When performing collation of an inputted fingerprint image and fingerprint data stored in advance, a fingerprint collation device generally extracts the directions of fingerprint ridges showing the fingerprint ridge directions in the fingerprint image and performs fingerprint collation processing based thereon. Through accurately extracting the directivity of the ridges contained within the fingerprint image, the accuracy of the fingerprint collation processing is improved. Thus, devices and methods for extracting the ridge directions have been disclosed.
As a related technique thereof, known is a ridge direction determining device which acquires the extreme value of the gradation fluctuation amount for a plurality of quantization directions set in advance within an image by utilizing the fact that the gradation fluctuation of pixels is small in the direction same as the direction of stripes (direction along the stripe lines) in an image with a stripe pattern and the gradation fluctuation of the pixels is large in the direction orthogonal to the stripes, and determines the direction of the stripes (stripe lines) based on the extreme value, i.e. determines the ridge direction (Patent Document 1). Further, as a related technique thereof, known is a device which determines the ridge direction by calculating a gradient vector in pixels which determine the ridge direction (Patent Document 2).
For extracting the ridge direction, it is common not to calculate the ridge directions in all the pixels but to extract the ridge direction in a small region unit of about 8×8 pixels for reducing the calculation amount.
With the above-described related techniques, the ridge direction is extracted locally. “Locally” herein means that the ridge direction is determined by using only the gradation of a pixel group in the vicinity of the target pixel, and the consistency with the ridge direction of the entire fingerprint is not taken into consideration.
Further, in Patent Document 2 described above, a region where the ridge directions are stable within a relatively wide area within an image is defined as a direction stable region, and a region where the directions change drastically such as the regions in the vicinity of the core and a delta is defined as a direction fluctuating region. Further, a region within an image where influence of noises such as wrinkles and blur is small so that extraction of the ridge direction is easy is defined as a high confidence region, and a region where influence of noises such as wrinkles and blur is large so that extraction of the ridge direction is difficult is defined as a low confidence region.
Thus, there may be chances of extracting wrong directions due to the influences of wrinkles and blur since extraction is done locally by using the gradation change only in the neighborhood pixel group with Patent Documents 1 and 2 described above. Further, there is also an inconvenience that a proper ridge direction cannot be extracted in a region where the ridge curvature is large such as in the vicinity of the core and the vicinity of the delta in a fingerprint image.
As a related technique thereof, there is known a method which performs smoothing of the ridge direction pattern for modifying the incorrectly extracted ridge direction (Patent Document 3).
Further, as a related technique thereof, there is disclosed a method which performs two-dimensional Fourier transformation in a local region where the ridge direction is to be defined, analyzes peaks of the Fourier transformation plane acquired as a result and, when there are a plurality of direction candidates (a plurality of peaks), determines the direction that exhibits a small confliction between the direction of the neighborhood region with an energy minimizing method. Further, there is also disclosed a method which expands an adaptable range through adapting the evaluation system of the energy minimizing method in the direction stable region and the direction fluctuating region (Patent Documents 4, 5).
Further, as a related technique thereof, there is disclosed a method which corrects the degree of confidence in the direction of the neighborhood region by using the ridge direction determined at a certain point, and determines the ridge direction of the neighborhood region by reevaluating the corrected degree of the direction confidence (Patent Document 6).
With Patent Document 6, extraction of the ridge direction exhibiting a small confliction is performed also in the direction fluctuating region by repeating the processing for reevaluating the degree of the direction confidence (direction propagation processing).    Patent Document 1: Japanese Unexamined Patent Publication Sho 52-97298    Patent Document 2: Japanese Unexamined Patent Publication Hei 8-7097    Patent Document 3: Japanese Unexamined Patent Publication Hei 5-181956    Patent Document 4: Japanese Unexamined Patent Publication 2002-288641    Patent Document 5: Japanese Unexamined Patent Publication 2002-288672    Patent Document 6: Japanese Unexamined Patent Publication 2007-65900
However, the smoothing processing of the ridge directions depicted in Patent Document 3 described above depends on the ridge directions locally extracted by the ridge direction extracting processing and the direction confidence data thereof. Thus, when the accuracy of the extracted ridge directions or the direction confidence data thereof is low, the accuracy in the smoothened ridge directions becomes low as well.
For example, when a wrong ridge direction is extracted in a wide range of region within an image due to the influence of the noises such as wrinkles within a fingerprint image, it is not possible to perform correction with the smoothing processing. Further, it is not possible to normalize (correct) the ridge directions extracted by the smoothing processing in the regions in the vicinity of the core and the delta where the ridge directions change drastically because the ridge curvature is large.
Further, the related techniques depicted in Patent Documents 4 and 5 described above exhibit a large effect in the direction stable region. However, there is a limit in the effect with the direction fluctuating region, so that a wrong ridge direction may be extracted. It is considered to be so because there is a limit for adapting the energy minimizing method to the direction fluctuating region.
For example, in a case where the ridge direction data that is extracted from the fingerprint image shown in FIG. 4 by using the method disclosed in Patent Document 4 and Patent Document 5 is superimposed on the original fingerprint image, the ridge direction within an elliptic broken-line region is extracted incorrectly as shown in FIG. 16. The broken-line region is a region including the center (core) that is the direction fluctuating region within the fingerprint image.
Further, the related technique depicted in Patent Document 6 described above exhibits a large effect on extracting a proper ridge direction in the direction fluctuating region. However, there is a limit in the effect with the low confidence region, so that a wrong ridge direction may be extracted.
This is because the degree of confidence of the noise direction in an image with the noises such as prominent wrinkles and blur is extremely higher than the degree of confidence of the ridge direction, so that it is not possible to determine (specify) the proper ridge direction only by correcting the degree of confidence.
For example, FIG. 17 shows an image in which the ridge direction data extracted by using the technique disclosed in Patent Document 6 described above is superimposed on the fingerprint image shown in FIG. 4.
FIG. 17 shows that the ridge directions extracted within the elliptic broken-line region contain the ridge direction extracted incorrectly (wrong extraction). The noise of wrinkles is prominent within the broken-line region, so that the degree of confidence indicating that the ridge direction is extracted properly is extremely low. Thus, it is not possible to perform rectification effectively by correcting the degree of confidence.
It is an object of the present invention to improve the inconveniences of the related techniques described above and to provide a ridge direction extraction device, a ridge direction extraction method, and a ridge direction extraction program, which are capable of accurately extracting the ridge direction in a fingerprint image regardless of the ridge shape.